fix(discov): avoid duplicate watch events and monitors,duplicate addKv

[YangXiaowu]

What problem does this PR solve?

This PR fixes two issues in etcd discovery watchers:

1. Duplicate etcd PUT events for the same key/value caused subscribers to
   repeatedly append the same key in the local container, which could lead to
   unbounded memory growth.
2. Multiple monitors for the same endpoints/key could start duplicate etcd
   watch goroutines instead of reusing the existing watcher.

What is changed?

• Ignore duplicate PUT events when the key already has the same value.
• When a key changes to a different value, notify listeners with
  OnDelete(old) before OnAdd(new).
• Use the previously stored value for delete notifications when available.
• Prevent repeated Monitor calls for the same watch key from starting
  duplicate watch streams.
• Add regression tests for duplicate PUT handling and duplicate monitor
  handling.

Tests

go test ./core/discov/...
go test ./zrpc/resolver/...